President Barack Obama’s re-election prospects look dimmer by the day, so — absent an economic turnaround — he’s likely to try to win in 2012 the way Harry Truman did in 1948: by blasting Republicans.

Every new poll contains bad political news for Obama, suggesting that if the election is a referendum on him, he’s headed for defeat.

So, he’s bound to emphasize that elections are a choice and charge that what Republicans are serving up — austerity, mean-spiritedness and retrenchment — will hurt most Americans, not help them.

Already in speeches this summer he’s been blaming “partisan brinksmanship” — meaning GOP opposition — for Congress’ failure to help put the economy back on track.

Presumably, he will lay out an ambitious jobs agenda in his address to Congress this week — he has nothing to lose — with every expectation that it will be dead on arrival among Republicans.

Then he can wail away at them as “do nothing” as Truman did the Republican-dominated 80th Congress.

Obama is in worse shape in the polls than Truman was at a comparable point in 1947. Obama’s Gallup poll approval rating is 39 percent. Truman’s was 55 percent, but it actually descended to 36 percent in April 1948, creating the near-universal perception that he would lose in November.

Unfortunately for Obama, the unemployment rate in 1947 and 1948 was 3.5 percent, and economic indicators were all improving despite labor strife and other difficult post-war adjustments.

Obama is burdened not just with a 9.1 percent unemployment rate but with the widespread fear that he and the federal government have no idea how to rekindle the economy and could not bring it off if they did know.

As Republican pollster Bill McInturff summarized the situation in a presentation last week, the collapse of consumer confidence is “chilling — not only for what it may portend for the economy but also for [Obama’s] re-election chances.”

The Michigan Consumer Sentiment Index hit 55.7 percent in August, down from 67.7 percent last October and 63.7 percent in July. “There is no precedent for an incumbent president being re-elected when this scale is at 75 or below,” McInturff said.

As Washington Post polling shows, only 33 percent of voters — Obama’s base — have confidence that the president can make the right decisions about the economy. Only 26 percent have confidence in the federal government.

The economic situation has eaten like acid into Obama’s public image. The Associated Press-GfK poll last month showed all-time lows for his ratings on strong leadership (51 percent), understanding the problems of ordinary Americans (53 percent) and even “caring about people like you” (55 percent).

He is even near his low for being likable, at 78 percent.

As many commentators have noted, Obama got elected as the ultimate national Rorschach test, invested with voters’ best hopes and greatest fears, with hopes giving him an upside.

Now he’s a Rorschach on the downside, with many liberals disappointed that he wasn’t the “transformational” figure they expected but a “compromiser” and with independents disappointed that he wasn’t a healer and unifier but a polarizer.

He got elected carrying 52 percent of independents, but his approval rating with them now is 38 percent, according to a Marist-McClatchy poll.

Among women, he’s down to 49 percent from 56 percent; among young voters, down to 52 percent from 66 percent; moderates, down to 49 percent from 60 percent; and even liberals, down to 75 percent from 89 percent.

The only good news for Obama is that Congressional Republicans are viewed more negatively than he is. His hope has to be that their presidential nominee can be closely tied to them.

The Washington Post poll shows confidence in Congressional Republicans to make good economic decisions is just 18 percent.

Almost all polls indicate that the public still blames former President George W. Bush more than Obama for the country’s economic plight — 52 percent to 32 percent in the latest Quinnipiac University survey.

On policy, the Marist-McClatchy poll showed voters believe far more in Obama’s policies than in the GOP’s.

Sixty-nine percent of voters favor reducing deficits by increasing taxes on those making more than $250,000 a year, including 51 percent of Republicans, 68 percent of independents and even 43 percent of tea party adherents.

In 1947, Truman proposed a very liberal “fair deal” agenda including richer unemployment, Social Security and public employee benefits, public works programs, a higher minimum wage, price controls and aid to farmers. The GOP rejected almost all of it.

Obama is signaling he’ll call for infrastructure building, patent reform, trade agreements and continued payroll tax cuts. I hope he also goes big on tax reform. Whatever he proposes, Republicans are sure to oppose most of it.

His hope has to be that Republicans nominate a candidate associated with the tea party — which has support from only 20 percent of voters, according to the Pew Research Center — or that they choose someone so unacceptable that the tea party runs a separate candidate.

What Obama can’t count on is that the GOP candidate will be so confident of victory that he says nothing, as Thomas E. Dewey did in 1948. Obama will have to fight, and I’d guess the election will be ugly.